Geoscience Reference
In-Depth Information
Figure 7.22.
Geotherms for the standard model shown in Fig. 7.3(a) for two cases.
(a) Erosion at 1 km Ma
−1
for 25 Ma, and then no further erosion. The dotted
horizontal line is the surface after erosion; grey shading shows material eroded.
(b) Deposition at 0.5 km Ma
−1
for 25 Ma. The dotted horizontal line is the original
surface before deposition; grey shading shows material deposited. Curve 0, the
standard equilibrium geotherm of Fig. 7.3(a); curve 25, the geotherm immediately
after erosion/deposition for 25 Ma; curve 100, the geotherm after 100 Ma; and
unlabelled dashed line, the final equilibrium geotherm. (After Nisbet and Fowler
(1982).)
profound effect on the geotherm (Fig. 7.22) and on the pressure-temperature
(
P
-
T
) path through which any metamorphic rock passes. The shape of the meta-
morphic geotherm, that is, the
P
-
T
trajectory inferred from the metamorphic
rocks exposed at the surface, is also strongly influenced by erosion and, as is
shown later, often does not at any time represent the actual geotherm.
The intrusion, burial and overthrust models of the previous sections are now
subjected to erosion and deposition to illustrate the effects of these processes.
Two erosion models are used for the burial and intrusion models: in space across
the model, the first has strong erosion of the country rock and deposition on the
trough, whereas the second has strong erosion of the trough and deposition on
the country rock. For the overthrust, the erosion is taken to be constant across the
model. All erosion rates decay with time.
The burial model with erosion
In the first erosional model, deposition occurs in the centre of the trough at an
initial rate of 1.1 km Ma
−
1
while erosion occurs at the edges of the model at
an initial rate of 3.3 km Ma
−
1
.Figure 7.23 shows this burial model after 20 Ma
when the sedimentary trough has been further covered by sediment and deep
erosion has taken place in the country rock. Figure 7.23 also indicates the maxi-
mum temperatures attained during the 20 Ma period by the rocks finally exposed
at the surface. These are the rocks available to a field geologist. This maximum
temperature is not necessarily preserved by the highest-grade minerals, but it is
sufficient here to assume that the mineral assemblages exposed at the surface are
